1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention relates to an electrophotographic image forming method and apparatus using a photoreceptor. Further, the present invention relates to a process cartridge for electrophotographic image forming apparatus, which includes a photoreceptor.
2. Discussion of the Background
As electrophotographic image forming methods, various methods using a photoreceptor such as the Carlson process and its modified processes are known and have been used for image forming apparatus such as copiers and printers. Among photoreceptors used for such image forming methods, photoreceptors using an organic photosensitive material have been currently used because of having advantages such as low manufacturing cost, good productivity and low pollution.
Specific examples of the organic photoreceptors include the photoreceptors including one of the following photosensitive layers:
(1) organic photoconductive resin layers typified by poly-N-vinylcarbazole;
(2) charge transfer complex type photosensitive layers as typified by a combination of poly-N-vinylcarbazole (PVK) with 2,4,7-trinitrofluorenon (TNF);
(3) pigment dispersion type photosensitive layers typified by a combination of phthalocyanine and a binder resin; and
(4) functionally-separated photosensitive layer typified by a combination of a charge generation material and a charge transport material.
Among these photoreceptors, the functionally-separated photoreceptors attract considerable attention now.
The electrophotographic image forming methods typically include the following processes:
(1) charging an electrophotographic photoreceptor in a dark place (charging process);
(2) irradiating the charged photoreceptor with imagewise light to form an electrostatic latent image thereon (light irradiating process);
(3) developing the latent image with a developer including a toner mainly constituted of a colorant and a binder to form a toner image thereon (developing process);
(4) optionally transferring the toner image onto an intermediate transfer medium (first transfer process);
(5) transferring the toner image onto a receiving material such as a receiving paper ((second) transfer process);
(6) heating the toner image to fix the toner image on the receiving material (fixing process); and
(7) cleaning the surface of the photoreceptor (cleaning process).
The mechanism of forming an electrostatic latent image in the functionally-separated photosensitive layer having a charge generation layer and a charge transport layer formed on the charge generation layer is as follows:
(1) when the photosensitive layer is exposed to light after being charged, light passes through the transparent charge transport layer and then reaches the charge generation layer;
(2) the charge generation material included in the charge generation layer absorbs the light and generates a charge carrier such as electrons and positive holes;
(3) the charge carrier is injected into the charge transport layer and transported through the charge transport layer, which is caused by the electric field formed by the charge on the photosensitive layer;
(4) the charge carrier finally reaches the surface of the photosensitive layer and neutralizes the charge thereon, resulting in formation of an electrostatic latent image.
For such functionally-separated photoreceptors, a combination of a charge transport material mainly absorbing ultraviolet light and a charge generation material mainly absorbing visible light is known to be useful.
Currently, a need exists for a photoreceptor having a long life. In particular, investigation of improving mechanical durability (i.e., abrasion resistance) of photoreceptors has been mainly made. For example, new binder resins have been proposed in Japanese Patent Publication No. (hereinafter referred to as JPP) 8-20739, etc. and various photoreceptors having new construction have also been disclosed. This is because the life of a photoreceptor substantially depends on the abrasion of the photosensitive layer and does not depend on the deterioration of the electrostatic properties of the photoreceptor.
However, when the abrasion resistance of photoreceptors is improved by various methods, it is considered that there will be severe demands for improving the deterioration of electrostatic properties such as decrease of charge potential (i.e., the potential of a dark area of a photoreceptor VD, hereinafter sometimes referred to as a dark area potential) and increase of residual potential (i.e., the potential of a lighted area of the photoreceptor VL, hereinafter sometimes referred to as a lighted area potential) In attempting to improve the deterioration of electrostatic properties, main materials constituting organic photoreceptors, such as charge generation materials and charge transport materials, have been improved. In addition, methods such that various additives such as antioxidants are added to photoreceptors have also been proposed. However, there is a trade-off between the deteriorated electrostatic properties, i.e., the decrease of charge potential and the increase of residual potential. Therefore, there is no method for improving both the decrease of charge potential and the increase of residual potential. Therefore, a photoreceptor having good combination of high dark area potential and low residual potential is earnestly desired.
As one of measures against abrasion of photoreceptors, methods in which a protective layer is formed on a surface of a photoreceptor have been proposed. Investigation of forming a protective layer is at first made for inorganic photoreceptors and has been disclosed in, for example, JPP 2-3171, 2-7058 and 3-43618. In these cases in which a protective layer is formed on inorganic photoreceptors, a filler having a relatively low resistance is preferably used for the protective layer. Therefore, when such photoreceptors are charged, the entire protective layer or the interface between the protective layer and the inorganic photosensitive layer is typically charged rather than the surface of the photoreceptor.
When a latent image is not formed on a surface of a photoreceptor but is formed on the inside of the protective layer, the photoreceptor has an advantage such that the resultant electrostatic latent image is hardly influenced by deficiencies of the surface of the photoreceptor, such as scratches. However, in order to impart a protection function to the protective layer, a large amount of an electroconductive filler such as metal oxides has to be added to the protective layer. In such a case, even if the protective layer is made so as to be transparent by using a suitable metal oxide, the resistance of the entire protective layer or the surface resistance of the protective layer decreases, resulting in occurrence of blurred images in repeated use. In attempting to solve the blurred image problem, JPP 2-7057 and Japanese Patent No. 2,675,035 have disclosed methods in which the concentration of an electroconductive metal oxide is changed in the depth direction of the protective layer.
In addition, in attempting to solve the blurred-image problem on the process side, a device including a heater heating a photoreceptor is proposed. By heating a photoreceptor, occurrence of blurred images can be avoided. However, when a drum heater is set in the photoreceptor, the diameter of the photoreceptor has to be widened.
Currently electrophotographic image forming apparatus are miniaturized more and more and therefore photoreceptors having a small diameter are mainly used. Since this heating technique cannot be used for such photoreceptors having a small diameter, it is hard to provide small-diameter photoreceptor having good durability. In addition, when a drum heater is provided in an image forming apparatus, the apparatus has many drawbacks such that the apparatus becomes large-sized; electric power consumption seriously increases; and it takes a long time until the apparatus is warmed up.
When a protective layer including a filler having a low electric resistance is formed as a surface layer on an organic photoreceptor (a so-called OPC) including a charge generation material and a charge transport material, a problem which occurs is that the resultant images have tailing when the photoreceptor is repeatedly used. In addition, the above-mentioned method in which the concentration of an electroconductive metal oxide included in a protective layer is changed in the depth direction of the protective layer and which is useful for inorganic photoreceptors is used for OPCs, almost the same results are produced (i.e., images having tailing are produced).
The reason is not yet determined, however it is considered to be that the current image forming methods in which dot images are written according to digital signals on the surface of a photoreceptor are largely different from the old image forming methods in which an inorganic photoreceptor is typically used (i.e., in which an analogue image is formed on an inorganic photoreceptor). Namely, the level of the requirement for resolution of latent images formed on a current photoreceptor, which is required from the machine side, is largely changed, and therefore the tailing problem may be noticeable.
When such situations are taken into consideration, it is essential to use a filler having a high resistance in a surface layer of an optical photoreceptor instead of a low resistance filler. However, when a filler having a high resistance is used, a problem such that residual potential of the resultant photoreceptor increases tends to occur. When residual potential increases (i.e., the lighted-area potential of a photoreceptor in an image forming apparatus increases), the image density and the half-tone reproducibility of the resultant images deteriorate. In attempting to solve such problems, the dark-area potential should be increased. However, when the dark-area potential is increased, the electric field strength also increases, resulting in production of image defects such as background development and shortage of life of the photoreceptor.
In attempting to avoid increase of residual potential, methods in which a photoconductive protective layer is formed have been disclosed in JPPs 44-834, 43-16198 and 49-10258. However, imagewise light is absorbed by the protective layer, and therefore the quantity of light which reaches the photosensitive layer decreases, resulting in decrease of the photosensitivity of the photoreceptor. Therefore, there is little effect.
Japanese Laid-Open Patent Publication No. (hereinafter JOP) 57-30846 discloses a method in which a metal or a metal oxide having an average particle diameter not greater than 0.3 xcexcm is included as a filler in a protective layer to prepare a transparent protective layer, resulting in prevention of increase of residual potential. However, its effect of preventing increase of residual potential is not insufficient, and therefore the problem cannot be solved.
This is because the increase of residual potential is caused by charge trapping due to the added filler and uneven dispersion of the filler rather than deterioration of charge generation efficiency. Even when a filler having an average particle diameter not greater than 0.3 xcexcm is used, the transparency of the resultant protective layer decreases if the filler aggregates. On the contrary, when a filler having an average particle diameter not less than 0.3 xcexcm is used, a transparent protective layer can be formed if the filler is uniformly dispersed.
In addition, JOP 4-281461 discloses a method in which a charge transport material is included in a protective layer together with a filler in attempting to prepare a photoreceptor capable of preventing increase of residual potential while having a good mechanical strength. To include a charge transport material in a protective layer improves the charge mobility and therefore the decrease of residual potential can be improved to some extent. However, when a filler is added, residual potential is remarkably increased, which is caused by the increase of resistance of the protective layer and the number of charge trap sites in the protective layer. Therefore, there is a limit to restraint of the increase of residual potential by this method the thickness of the protective layer has to be decreased or the filler content has to be decreased. Accordingly, the demand for a photoreceptor having good durability cannot be satisfied.
As other methods for curbing the increase of residual potential, a method in which a Lewis acid is included in a protective layer (JOP 53-133444); a method in which an organic proton acid is included in a protective layer (JOP 55-157748); a method in which an electron accepting material is included in a protective layer (JOP 2-4275); and a method in which a wax having an acid value of 5 mgKOH/g is included in a protective layer (JOP2000-66434), have been disclosed.
These methods improve the charge injection at the interface between the protective layer and the charge transport layer. It is considered that by these methods portions having a low resistance are formed in the protective layer, and the charge can reach the surface of the protective layer, resulting in decrease of residual potential. However, the resultant images produced by these methods tend to be blurred. In addition, when an organic acid is included in a protective layer, the dispersion of the filler in the protective layer tends to deteriorate. Thus, these methods produce adverse effects, and therefore it can be said that the problem cannot be solved.
In photoreceptors in which a filler is included to improve their durability, it is needed to avoid production of blurred images and to curb increase of residual potential, in order to produce high quality images. In addition, it is also important that charges in a photoreceptor linearly move toward the surface of the photoreceptor without being obstructed by the filler included in the protective layer. Therefore, it is needed that the filler in the protective layer is well dispersed therein. When the filler included in a protective layer agglomerates, movement of the charges injected into the protective layer from the charge transport layer are obstructed by the filler when the charges move toward the surface of the protective layer. Therefore a toner image constructed of scattered toner particles is formed, resulting in deterioration of resolution of the toner image.
In addition, when imagewise light irradiates such a protective layer including an agglomerated filler, the light is scattered by the filler, resulting in deterioration of light-transmittance, and thereby resolution of the resultant image deteriorates.
Further, the dispersion of a filler included in a protective layer largely influences the abrasion resistance of the photoreceptor. When a filler seriously agglomerates (i.e., a filler is poorly dispersed), the abrasion resistance of the resultant photoreceptor deteriorates. Therefore in order to provide a photoreceptor in which a filler is included in a protective layer to improve the durability of the photoreceptor and which can produce high quality images, it is important not only to prevent occurrence of blurred images and increase of residual potential but also to improve dispersion of the filler in the protective layer.
However, a solution by which these problems are solved at the same time has not been discovered. Namely, when a filler is included in a surface layer of a photoreceptor to improve its durability, blurred images tend to be produced and residual potential tend to increase, and therefore a problem in that high quality images cannot be obtained remains. As mentioned above, a drum heater should be provided in an image forming apparatus to improve such a problem. However, a drum heater cannot be installed in a small-sized photoreceptor, which is earnestly desired to have good durability. Therefore there is no small photoreceptor having good durability and capable of producing high quality images. To install a drum heater is an obstruction to miniaturized image forming apparatus and image forming apparatus having low electric power consumption.
Currently organic photoreceptors have advantages against inorganic photoreceptors, such as high photosensitivity, wide spectral photosensitivity, low pollution, and good electrostatic durability. In order to make full use of such advantages, the mechanical durability and electrostatic durability of the organic photoreceptors have to be improved. In addition, in order to develop an image forming apparatus having good durability, a photoreceptor which can produce high quality images while having good durability is especially desired.
Accordingly, an object of the present invention is to provide a photoreceptor which can produce high quality images and which has good durability. Specifically, an object of the present invention is to provide a stable photoreceptor which has good mechanical durability and electrostatic durability (i.e., increase of residual potential and occurrence of blurred images can be curbed) and which can produce high quality images even in repeated use.
Another object of the present invention is to provide a photoreceptor which has stable photosensitive properties even when environmental conditions such as temperature and humidity change and which is resistant to reaction gases such as ozone and NOx.
Yet another object of the present invention is to provide an image forming method which uses the photoreceptor mentioned above and by which high quality images can stably produced for a long period of time.
A further object of the present invention is to provide a small-sized image forming apparatus and a process cartridge by which high quality images can be stably produced for a long period of time without frequently changing the photoreceptor.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an electrophotographic photoreceptor including at least an electroconductive substrate, a photosensitive layer formed on the substrate and a protective layer formed on the photosensitive layer and including a binder resin, wherein when a solution in which the binder resin is dissolved in an organic solvent incompatible with water is mixed with deionized water having an electroconductivity not greater than 1 xcexcS/cm and substantially the same weight as that of the solvent while being agitated, the water has an electroconductivity not greater than 2 xcexcS/cm.
It is preferable that the binder resin is previously subjected to a refinement treatment such as washing treatments using an alkali and/or an acid to remove ionic impurities therefrom.
The binder resin preferably includes a polycarbonate resin.
The protective layer (i.e., the surface layer) preferably includes a filler. The filler is preferably an inorganic pigment having a specific resistance not less than 1010 xcexa9xc2x7cm. The inorganic pigment is preferably a metal oxide selected form the group consisting of silica, alumina and titanium oxide.
The pH and dielectric constant of the inorganic pigment are preferably not less than 5 and not less than 5, respectively. The surface of the inorganic pigment is preferably subjected to a treatment preferably using a material selected from the croup consisting of titanate coupling agents, aluminum coupling agents, higher fatty acids, alumina, titanium dioxide and zirconium dioxide, and their mixtures and their mixtures with a silane coupling agent with at least one of the materials mentioned above. The ratio (Ws/Wf) of a weight (Ws) of the surface treating agent to a weight (Wf) of the filler is from 0.03 to 0.30. The primary particle diameter of the filler is preferably form 0.01 to 0.5 xcexcm.
The protective layer preferably includes a charge transport material. The charge transport material is preferably a charge transport polymer material such as polycarbonate resins having a triarylamine structure in its main and/or side chain.
The polycarbonate resin preferably has a repeating unit having the following formula (A) or (B): 
wherein R1 and R2 independently represent a hydrogen atom, a substituted or unsubstituted aliphatic group, a substituted or unsubstituted carbon ring or a substituted or unsubstituted aromatic group; and R3 to R10 independently represent a hydrogen atom, a halogen atom, a substituted or unsubstituted aliphatic group or a substituted or unsubstituted carbon ring, and 
wherein R3 to R10 independently represent a hydrogen atom, a halogen atom, a substituted or unsubstituted aliphatic group or a substituted or unsubstituted carbon ring; and Z represents a substituted or unsubstituted carbon ring or an atom group needed for forming an unsubstituted heterocyclic group.
The repeating unit is preferably one of the following repeating units (1) to (3): 
In another aspect of the present invention, an image forming method is provided which includes the steps of charging the photoreceptor of the present invention, irradiating the charged photoreceptor with light to form an electrostatic latent image, developing the latent image with a developer to form a toner image on the photoreceptor, and transferring the toner image onto a receiving material. The light irradiation process is preferably digitally performed (i.e., dotted light images are formed on the photoreceptor by irradiating a light beam) using a laser diode (LD) or a light emitting diode (LED) as a light source.
The image forming method preferably further includes a step of applying zinc stearate on the surface of the photoreceptor. In addition, the toner preferably includes zinc stearate. Further, it is preferable that when the above-mentioned image forming process are not performed, a cleaning process including the steps of adhering the toner on the surface of the photoreceptor at the developing section and collecting the toner at the cleaning section is performed.
In yet another aspect of the present invention, an image forming apparatus is provided which includes the photoreceptor of the present invention, a charger configured to charge the photoreceptor, a light irradiator configured to irradiate the charged photoreceptor with light to form an electrostatic latent image thereon, an image developer configured to develop the latent image with a developer to form a toner image thereon, and a transfer device configured to transfer the toner image onto a receiving material.
Preferably the charger is a contact charger or a proximity charger. A DC voltage overlapped with an AC voltage is preferably applied to the charger. It is preferable that the light irradiator digitally writes light images using a laser diode (LD) or a light emitting diode (LED) as a light source. Further it is preferable that the image forming apparatus further has a lubricant applicator applying a lubricant such as stearic acid to the photoreceptor.
In a further aspect of the present invention, a process cartridge for an image forming apparatus is provided which includes the photoreceptor of the present invention, a housing and at least one of a charger, an image irradiator, an image developer, an image transferee, a cleaner and a discharger.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.